For a big rock that happens to be trapped in our planet’s orbit, the moon certainly has a lot of pull – literally – on life here on Earth, from the tides to centuries of art, religious beliefs and folklore. We can’t help but be fascinated by its beautiful glow, its changes throughout the month, its movement across the sky and the vast rocky landscape on its surface.

The Full Moon

This stunning image shows the moon as it was captured by the Galileo spacecraft in 1992. The moon is ‘full’, appearing perfectly round, when it is on the opposite side of the earth from the sun. This is the only time when the back half of the moon is truly dark. (See this image large!)

Lunar Eclipse

The moon can take on an eerie red glow in the midst of an eclipse. Lunar eclipses occur when the moon is perfectly aligned behind the earth, with the earth blocking the sun. The phenomenon can be viewed by anyone on the side of the earth facing the moon when it occurs, and can last several hours. In contrast, a solar eclipse occurs when the moon blocks the sun as viewed from the earth; solar eclipses last just a few minutes.

Waxing and Waning

As the moon passes through its various stages, its shape appears to change. However, this is only an optical illusion based on the relative location of the moon to the earth and the sun. When the illuminated side of the moon is on the right, the moon is ‘waxing’ or building toward a full moon. When it’s on the left, the moon is ‘waning’ toward the ‘new moon’, when the moon is in total darkness.

Crescent Moons

Crescent moons occur both at the waxing and waning phases, when the moon is between 1-49% visible. Waxing crescent moons are visible between about 3pm and post-dusk, while waning crescent moons can be seen between pre-dawn and 9am. At 50% visible, it’s a ‘quarter moon’, and when the moon is between 51-99% visible in either waxing or waning phases , it’s referred to as ‘gibbous’.

Earthshine

What causes that strange glow on the dark side of the moon? Often called ‘earthshine’, this glow comes from sunlight reflected by the earth. It occurs when the light from the sun is reflected from the surface of the earth to the moon and then back again to our eyes. It can be seen most clearly during the crescent phases.

The Moon Beside the Earth

Unlike most other planets (aside from Pluto, which technically isn’t even a planet anymore), Earth’s moon is relatively large compared to the size of its planet. It’s a quarter of the diameter of the earth, and 1/81 its mass. It takes the moon about 29.5 days to orbit the earth; this time period was the basis of what we now use to divide the days of the year into months.

Lunar Craters

These images capture some of the moon’s craters including Goclenius, Daedalus and Tycho. The word ‘crater’ was coined by Galileo from the Latin word for cup. They were formed by the impact of meteors and asteroids. The lack of water, atmosphere and tectonic plates on the moon mean there is little erosion, preserving the crates for millennia.

The ‘Seas’ of the Moon

Centuries ago, astronomers believed that the dark, featureless areas on the moon that can be seen with the naked eye were seas, hence the term ‘mare’ (plural maria) used to identify these areas. We now know that these plains are solidified pools of ancient basaltic lava which flowed into the depressions associated with impact basins between 4.2 and 1.2 billion years ago.

The Moons of Jupiter

Of course the earth’s moon is not alone in its beauty; many other planets have moons that are just as incredible. Jupiter has the most moons of any planet, with 64 confirmed. The largest of them are the four ‘Galilean moons’, discovered in 1610 by Galileo Galilei; this observation marked the first time objects were found to orbit a body that was neither the sun nor the earth. From left to right, the four Galilean moons as depicted above are Io, Europa, Ganymede and Callisto.

The Moons of Saturn

Saturn is home to the second largest moon in our solar system, know as ‘Titan’. Titan is larger than the planet Mercury and has an Earth-like atmosphere with hydrocarbon lakes and networks of dry rivers. In addition to Titan, Saturn has 61 moons of vastly variable sizes; 38 of them are ‘irregular satellites’, likely captured minor planets or collections of space debris.